1. Field of the Invention
This invention relates to pulse radar, particularly to pulse doppler radar, which may be used in a search, track, or mapping mode.
2. Description of the Prior Art
Present pulse doppler radar systems transmit one or two frequencies to detect moving targets such as an aircraft against a background of ground terrain or sea. A single frequency radar has one transmitter which can generate an instantaneous peak power output and an average power output to the antenna. The frequency source for the transmitter is generated by mixing the signals from a stable local oscillator (STALO) and from a coherent oscillator (COHO) followed by selecting one of the frequency sidebands with a frequency filter. The most expensive subsystems of the radar are the transmitter and the stable local oscillator. The transmitter involves costly components and stringent requirements on frequency stability for the radar to maintain its ability to suppress ground clutter echoes with a moving target indicator (MTI). The stable local oscillator cost is directly related to the specified MTI capability to remove ground clutter echoes.
In a single frequency pulse doppler radar, the target radar cross section of an aircraft fluctuates slowly, caused by small changes in heading of the aircraft and its motion relative to the radar site. In order to achieve a high detection probability with such a fluctuating target, it is necessary to radiate sufficient power to detect the echo at those times when the radar cross section is much smaller than average. Another problem in a single frequency radar is that no MTI output is produced when a target moves toward or away from the radar a distance equal to an integral number of half wavelengths of the transmitted frequency during the interval between pulses. Therefore the radar cannot see targets at a number of fixed radial velocities or "blind speeds" since there is no output from the MTI.
In a two frequency pulse doppler radar, a single antenna is used to transmit and receive two frequencies, greatly reducing the blind speed problem. The remainder of the equipment associated with the radar is duplicated for each frequency; for example, two transmitters, mixers, STALO'S, COHO'S, synchronizers, code generators and duplexers are required. In addition, two diplexers and two receivers are required. The duplication of the equipment for two frequencies is the major disadvantage of present diplex operation, especially for the transmitter and the stable local oscillator which is expensive to build to the required frequency stability in order to obtain a specified MTI capability.